I'm trying to trouble shoot my son's truck long distance (He is in Suffield,Connecticut going to Tech school) and need some help. The truck is a bone stock '79 k10, 350, 4bbl quadrojet, 350 auto, 203TC. The problem is when cruising along, it backfires thruogh the carb. Not a real big BOOM, just a slight pop. It doesn't do this while accelerating, or while pulling a long hill, just when cruising at 30-40 MPH in town, less at highway speeds but still does it occasionally. The engine has 98k on it, I have replaced the distributor (reman unit) cap, rotor, plug wires, plugs, set timing, ALL vacuum hoses, replaced the fuel filter and "autopsied" the old one (paper element) Filter had some fine black debris and tiny red particles but was otherwise not what I would consider bad. Also has new fuel pump. Tonight I had him spray the carb base gasket with carb cleaner while it was running, idle speed didn't change at all. (I was hoping for a leaky base gasket) The carb is one of the "smog" ones that doesn't have the idle mixture screws, just plugs where the screws should be so I can't have him adjust mixture at idle. He has run 3 cans of drygas, 1 can of seafoam, and 1 bottle of STP fuel cleaner through it in the past 2 weeks. The problem has not changed at all from before, during, or after all of the above. I had him unplug the EGR valve vacuum line and plug it. I'm hoping that tommorow he'll call and tell me this helped but I'm running out of ideas. I think this is a "lean" condition causing the problem but obviously I'm unsuccesful so far . Anybody see anything I've overlooked or advise on what the problem may be. I'm thinking of a remanned carb next, but don't want to throw money at it beyond the basic tuneup parts I've done so far.